This application claims the priority of Japanese Patent Application No. 6-183446 filed on Aug. 4, 1994.
1. Field of the Invention
The present invention relates to an active material powder for non-sintered nickel electrodes of alkaline batteries such as nickel-hydrogen battery and nickel-cadmium battery and a process for producing the same, and to a non-sintered nickel electrode for alkaline batteries and a process for producing the same.
2. Description of the Prior Art
The known nickel electrode for alkaline batteries has been prepared by impregnating a substrate (sintered substrate) obtained by sintering nickel powder onto a perforated steel plate or the like, with an active material.
This type of nickel electrode is known as "sintered nickel electrode". With this sintered nickel electrode, in which particles of the nickel powder bonds together only weakly, so that increasing the porosity of the substrate causes the nickel powder to drop off from the substrate. The maximum porosity of the substrate practically attained has therefore been 80%. The sintered nickel electrode has the problem of the active material having a low packing density, since the substrate such as periorated steel plate should be provided and it cannot have a sufficiently high density.
Besides, it becomes necessary, in order to pack the sintered substrate with an active material, to employ a solution immersion process which requires repeating a complex step several times. This is because the sintered body formed by sintering has too line holes having a diameter of not more than 10 .mu.m.
To solve the above problems, there is available what is known as "paste-type nickel electrode", which is obtained by impregnating or coating a sintered alkali-resistant metal fiber or a carbon fiber nonwoven fabric or the like plated with an alkali-resistant metal, with a slurry or paste containing nickel hydroxide (active material), a binder and solvent. However, the paste-type nickel electrode, which contains the binder and permits a conductive network to form only to a small extent, has the problem of markedly low rate of utilization of the active material.
Japanese Patent Application Laid-open Nos. 234867/1987 and 237667/1987 propose, in order to increase the utilization rate of the paste-type nickel electrode, increasing the conductivity of the surface of the active material particles by covering the surface of the nickel hydroxide particles with cobalt hydroxide. The cobalt hydroxide then dissolves in the alkaline electrolyte used and forms monovalent HCoO.sub.2 -ion, which is further converted into highly conductive CoOOH (cobalt oxyhydroxide) at high voltage and deposits on the surface of the nickel hydroxide particles. Likewise, Japanese Patent Application Laid-open No. 62457/1991 proposes a process of covering the surface of nickel hydroxide particles with a solid solution film of nickel hydroxide and cobalt hydroxide.
However, with the paste-type nickel electrodes obtained by the above processes, in which the cobalt hydroxide covering the surface of nickel hydroxide particles diffuses into the particles in the course of repeated charge-discharge cycles, cannot maintain over a long period of charge-discharge cycles the inherent function of cobalt hydroxide of increasing the conductivity of the surface of the electrodes. As a result, it has been very difficult to obtain a paste-type nickel electrode with the discharge capacity decreasing only to a small extent in the course of charge-discharge cycles.